The sudden appearance of purple flowers in a lawn during late winter or early spring often signals the presence of a cool-season annual weed. These plants germinate in the fall, survive the winter, and become highly visible as they flower before the turfgrass fully greens up. This early display of purple is usually due to one of two very common members of the mint family. Understanding the specific identity of the plant is the first step toward effective management.
Common Purple Weeds in the Lawn
The two most frequent culprits are Henbit (Lamium amplexicaule) and Purple Deadnettle (Lamium purpureum), both of which possess the characteristic square stems common to the mint family.
Henbit is distinguished by its rounded, scalloped leaves that appear to clasp or wrap directly around the main stem, particularly the upper leaves near the flowers. Its pink-to-purple flowers emerge in whorls from the leaf axils along the stem.
Purple Deadnettle, in contrast, has triangular or heart-shaped leaves attached to the stem by a short leaf stalk, or petiole. The leaves at the top of the plant are frequently tinged deep red or purple, giving the weed its name. While both produce small, tubular, purplish-pink flowers, the leaf structure is the most reliable way to tell them apart.
Conditions That Encourage Growth
These winter annual weeds thrive in areas where the turfgrass is thin, weak, or dormant during the cooler months of the year. Their seeds germinate in late summer or early fall when soil temperatures drop, often between 50 and 60 degrees Fahrenheit. The weeds then form small rosettes that overwinter, giving them a head start on the lawn in spring.
Compacted soil or areas with excessive moisture also create a welcoming environment for these weeds. Thin turf density is the most significant factor, as it allows sunlight to reach the soil surface, which is necessary for seed germination. Any bare patch or area of stressed grass is an open invitation for these prolific seed producers.
Cultural and Manual Removal
Preventative and non-chemical strategies focus on creating a dense, healthy turf that naturally outcompetes the weeds. Since Henbit and Purple Deadnettle are annuals, preventing them from setting seed in the spring is an effective long-term control method. For small, isolated patches, manual removal is effective due to their relatively shallow, fibrous root systems, but this must be done before the flowers mature and release seeds.
Raising the mower blade height to the highest setting appropriate for the grass type helps to shade the soil, which discourages the germination of new weed seeds. Improving turf density through proper fertilization, aeration, and overseeding in the fall is a long-term solution. Aeration reduces soil compaction while overseeding fills in bare spots.
Selecting the Right Herbicide
Chemical control is most effective when timed correctly, targeting the weed’s life cycle. Pre-emergent herbicides are designed to prevent seed germination and must be applied in late summer or early fall, typically when night temperatures consistently drop below 60 degrees Fahrenheit. These products establish a chemical barrier in the soil that stops the weeds from emerging the following spring.
For existing, actively growing weeds, a post-emergent herbicide is necessary. Products known as “three-way” broadleaf weed killers are often recommended, containing active ingredients like 2,4-D, dicamba, and mecoprop (MCPP). These selective herbicides target the broadleaf weeds without damaging the turfgrass. Post-emergent applications are most successful in the fall when the weeds are small, or in early spring before the plants flower and die off naturally.